PPO (Proximal Policy Optimization) is a reinforcement learning algorithm that balances simplicity, stability, sample efficiency, general applicability, and strong performance. PPO replaced TRPO (Trust Region Policy Optimization) as the default algorithm at OpenAI due to its simpler implementation and greater computational efficiency, while maintaining comparable performance. PPO approximates TRPO by clipping the policy gradient and using first-order optimization, avoiding the computationally intensive Hessian matrix and strict KL divergence constraints of TRPO. The clipping mechanism in PPO constrains policy updates, prevents excessively large changes, and promotes stability during training. Its surrogate objectives and clip function enable the reuse of training data, making PPO sample efficient, especially for complex tasks.